DC motors have generally been utilized in variable speed applications because of the ability to accurately control such motors over a broad range of speeds and conditions. In DC motors the winding current controls the motor torque and can be directly measured to achieve accurate control for the desired operation.
In an AC induction motor the torque is a function of induced current in the rotor which in turn is a function of slip, i.e., the difference in speed between the rotor and the rotating magnetic field produced by the stator. The speed of the rotating magnetic field is determined by the frequency of the winding energizing current. However, due to the slip, the rotor speed differs therefrom by a variable amount related to the torque demands on the motor. Accurate control of rotor speed of an AC induction motor is difficult to achieve under variable torque conditions. Thus, even though AC induction motors are considerably less expensive than DC motors, they have generally not been used where accurate speed control is required.
One prior approach to servo speed control of induction motors is the "Transvector control" approach described, for example, in U.S. Pat. Nos. 3,593,083 and 3,824,437. Servo control of the induction motor is achieved by either sensing the magnetic field conditions in the motor airgap or in deriving the field vector values from the stator voltage and current vectors. An inverter is then controlled in accordance with the field vector values to supply an energizing signal to the motor having the desired phase, frequency and amplitude. Although this system functions well at running speed under load, this approach is characterized with poor control at low speeds. Under such conditions the magnetic fields in the motor are relatively weak or nonexistent and difficult to sense accurately. The calculated field vectors require integrations and therefore do not provide useful control information at zero speed. As a result, effective control based on the field vectors cannot be achieved at low speeds. Furthermore, excessive power usage at low speeds results in undesirable heating.
Another approach is disclosed in patent application Ser. No. 297,809 filed Aug. 31, 1981 by James S. Whited wherein slip factors are empirically determined for a particular motor and these slip factors are utilized to generate a synthesized sine wave energizing signal having the slip and amplitude required to produce the torque necessary for achieving servo speed control. The slip is a function of the difference between the desired speed and the actual speed, i.e., the velocity servo error. This approach eliminates the need for sensing or calculating the magnetic field vectors and provides effective control under load at running speed.
The later system, however, does experience some instability problems when operating under light loads. These problems are cured using techniques described in a companion application filed by the same inventors concurrently herewith and entitled "Control for Improving Induction Motor Transient Response." (403-46) The later approach also suffers from some control instability at higher speed, particularly in the range above the normal rated speed for the motor.
An object of this invention is to eliminate the aforementioned instability at higher speeds.
Another object is to provide an induction motor control system capable of effectively controlling speed over a range from zero to several times the rated speed of the motor.